The present invention relates to an improved method for setting the lifetime of charge carriers in semiconductor bodies for semiconductor devices by the formation of recombination centers in the semiconductor body.
The directed and reproducible setting of the lifetime of charge carriers plays a very important part in the semiconductor art since some properties, such as the frequency limit of planar transistors or the recovery time of thyristors, which properties codetermine the behavior of the semiconductor devices with respect to their dynamic characteristics, are dependent on the lifetime of the charge carriers. It is known that the lifetime of the charge carriers can be reduced, for example, by the diffusion of gold or platinum into a silicon semiconductor body.
Insofar as there exists the requirement of maintaining the recovery time of thyristors within a well defined range, the problem also arises of setting the concentration of the recombination centers in the semiconductor to a defined value. This means, for example, that for high frequency thyristors in which the recovery time is to lie in a range approximately between 5 and 50 .mu.sec, a gold concentration between about 10.sup.12 and 10.sup.14 atoms .sup.. cm.sup..sup.-3 has to be provided in the silicon body.
The directed and controlled setting of such small concentrations as to quantity, and possibly also sufficient separation according to type of atom, has been unattainable thus far with economically justifiable expenditures. This is so because for a diffusion of recombination centers from the surface it is necessary to have the surface covered with only a very thin layer of the recombination center forming material or substance. For example, for a silicon wafer of the conventional thickness of about 0.3 mm, the surface layer of the material before the diffusion process should be only about 10.sup.11 to 10.sup.12 atoms .sup.. cm.sup..sup.-2, if the concentration of the recombination centers upon completion of the diffusion and with assumed uniform distribution is to reach the above-mentioned values of some 10.sup.12 to some 10.sup.13 atoms .sup.. cm.sup..sup.-3.
Such a thin surface layer in the order of magnitude of 10.sup.11 to 10.sup.13 atoms .sup.. cm.sup..sup.-2 has been unattainable up to now with any of the known and conventionally applied methods. Neither by vapor-deposition nor by chemical or electrolytical means, for example, by deposition from an aqueous solution and subsequent tempering or heating, is it possible to produce such surface coating densities. The devices produced according to such methods practically always exhibit a soft blocking characteristic and thus reduced blocking capability. Presumably the reason for this reduced blocking capability is the particular type of gold diffusion.
The diffusion of, for example, gold into silicon is known to take place according to the Frank-Turnbull Mechanism. (F. C. Frank, D. Turnbull, Phys. Rev. 104, 617 (1956), see also W. M. Bullis Sol. St. Electron. 9, 143 (1966)) According to this mechanism, the gold atoms diffuse initially to the interstitial locations and are then converted, by deposition in vacancies, into substitutionally bound atoms. An equilibrium in concentration between vacancies in the lattice, the interstitial locations and the substitutionally bound atoms is thus produced. Only the gold atoms which are substitutionally incorporated into the lattice have an electrically active effect in this equilibrium, i.e. act as recombination centers.
If during the diffusion process more gold is introduced into the silicon than is required to fill the vacancies, the excess gold must remain at the interstitial locations, where it may possibly be discharged again upon cooling in the form of lumps at crystal dislocations. It is suspected that the reduced blocking capability is a result of this excess gold in the interstitial locations or deposited at the crystal dislocations. Thus there are reasons not to increase the concentration of the gold to the limit of its solubility, which in silicon, for example, lies at about 10.sup.16 atoms .sup.. cm.sup..sup.-3. With such a high gold concentration, the lifetime of the charge carriers would moreover become too short.